visual studio – What does C++ _Unfancy do?

I’m using Visual Studio 2022 and C++
In some code I see _Unfancy(...); and also _Unfancy(_Allocate_at_least_helper(...));

Visual studio documentation can not help me on _Unfancy, can’t find anything on this subject.
‘The C++ programing language, 4th ed, C++11’ does not mention it.
Search in cplusplus.com does not provide me with some explanation or documentation.
Hits in google on _Unfancy are not many and all what I see is indirect.

Are _Unfancy(...) and _Allocate_at_least_helper(...) microsoft specific calls? Is there any documentation on what it is suppose to do and how/when to use it?

Kind regards.

Addition after the reply from @heap underrun:

Yes, I’m browsing in microsoft headers. And microsoft is kind enough to stick to the rules and use an ‘_’ but not kind enough to tell what it is doing. Fortunately in my configuration and for the use that I’m looking at it is relative simple. I was curious about the calls but I think that the reply by @heap underrun clarifies the lack of documentation (and also that these kind of calls are not portable).